The present invention relates to a diagnostic apparatus for nuclear medicine which is capable of so-called SPECT (Single Photon Emission Computed Tomography) for measuring, as a photon number, gamma rays emitted from radioactive isotopes (RIs) injected into a human subject and reconstructing an internal distribution (RI distribution) of the RIs on the basis of the measured value.
The performance of the SPECT depends upon the elements inherent in the apparatus, such as the detection sensitivity, non-uniformity of the detection sensitivity and spatial resolution as well as variation elements, such as an internal scattering and non-uniformity of the internal absorption.
The present invention is associated with the non-uniformity of an internal absorption. Known as the highest accurate method out of those methods for correcting the non-uniformity of the internal absorption is a method for locating a radiation source of nuclides the same as those injected into the human subject relative to a detection and measuring gamma rays which have been emitted from the radiation source and transmitted through a human subject. That is, the method comprises counting the transmitted gamma rays as a photon number, reconstructing a TCT (transmission computed tomography) corresponding to an absorption coefficient map on the basis of the counted value and correcting an RI distribution in accordance with the TCT or an absorption coefficient map from the TCT.
However, this method requires separate operations, that is, a count operation for the TCT and count operation for the SPECT. Thus, the total time of the count operations becomes very long.
A method for reducing the total time of these count operations has recently been developed. This method comprises performing a count operation for the TCT together with the count operation for the SPECT. According to this method, the total time of the count operations need only take about one half that required for the TCT or SPECT. It is, however, necessary to give such a design consideration that these count values, one for the TCT and one for the SPECT, are prevented from being mixed with each other.
In order to prevent the mixing of the count value for the TCT and count value for the SPECT, use is made, as the nuclide for the TCT, a nuclide which is different in photoelectric peak from the nuclide for the SPECT. For example, use is made, as the nuclide for the TCT, of .sup.153 Gd whose photoelectric peak is about 100 keV and, as the nuclide for the SPECT, of .sup.201 T1 whose photoelectric peak is about 70 keV. By doing so it is possible to selectively separate the photons for the TCT and those for the SPECT in accordance with their energies.
If, in actual practice, however, the RI distribution is corrected with the TCT, an artifact (false image) is generated in the corrected RI distribution. The inventors have specified one cause as the cause of the artifact.
That cause is as follows. That is, a collimator made of lead is mounted on the detector so as to collimate gamma rays. When the gamma rays from .sup.153 Gd in the radiation source produce a photoelectric effect in the collimator, electrons at the K shell are principally produced. And characteristic X-rays (K-X rays) are generated incidental to the electrons at the K shell and some of them is output outside the collimator.
The photoelectric peak of the K-X rays is about 75 keV, a level very near to that of .sup.201 T1 for the SPECT. For this reason, the K-X rays will pass through an energy window (64-78 keV) for the SPECT which is centered in the photoelectric peak of the K-X rays. As a result, the number of photons passed through the energy window for the SPECT becomes a sum of the number of photons from .sup.201 T1 for the SPECT plus the number of photons of the K-X rays passed through the energy window for the SPECT. That is, the number of the photons from the .sup.201 T1 for the SPECT was not able to be calculated with high accuracy.